> On 07/12/2011 02:22 PM, harsha.jaga...@amd.com wrote: > > We would like to propose changing AVX generic mode tuning to generate > 128-bit > > AVX instead of 256-bit AVX. > > You indicate a 3% reduction on bulldozer with avx256. > How does avx128 compare to -mno-avx -msse4.2?
We see these % differences going from SSE42 to AVX128 to AVX256 on Bulldozer with "-mtune=generic -Ofast". (Positive is improvement, negative is degradation) Bulldozer: AVX128/SSE42 AVX256/AVX-128 410.bwaves -1.4% -1.4% 416.gamess -1.1% 0.0% 433.milc 0.5% -2.4% 434.zeusmp 9.7% -2.1% 435.gromacs 5.1% 0.5% 436.cactusADM 8.2% -23.8% 437.leslie3d 8.1% 0.4% 444.namd 3.6% 0.0% 447.dealII -1.4% -0.4% 450.soplex -0.4% -0.4% 453.povray 0.0% -1.5% 454.calculix 15.7% -8.3% 459.GemsFDTD 4.9% 1.4% 465.tonto 1.3% -0.6% 470.lbm 0.9% 0.3% 481.wrf 7.3% -3.6% 482.sphinx3 5.0% -9.8% SPECFP 3.8% -3.2% > Will the next AMD generation have a useable avx256? > I'm not keen on the idea of generic mode being tune > for a single processor revision that maybe shouldn't > actually be using avx at all. We see a substantial gain in several SPECFP benchmarks going from SSE42 to AVX128 on Bulldozer. IMHO, accomplishing even a 5% gain in an individual benchmark takes a hardware company several man months. The loss with AVX256 for Bulldozer is much more significant than the gain for SandyBridge. While the general trend in the industry is a move toward AVX256, for now we would be disadvantaging Bulldozer with this choice. We have several customers who use -mtune=generic and it is default, unless a user explicitly overrides it with -mtune=native. They are the ones who want to experiment with latest ISA using gcc, but want to keep their ISA selection and tuning agnostic on x86/64. IMHO, it is with these customers in mind that generic was introduced in the first place. Thanks, Harsha